It is conceivable that the battery mounted in an electric vehicle is used in the cold district, and a battery electrolyte might freeze while the battery is unused.
When the temperature of the battery lowers, although a battery charge state SOC does not lower, a usable input/output power for the battery decreases due to an increase of an internal resistance. Further, when the battery electrolyte freezes, the usable input/output power of the battery finally becomes 0 (zero), then in a case of the electric vehicle that uses the battery as a travelling energy source, the electric vehicle gets into a travel-impossible state.
Thus, the battery temperature control device, which controls the battery temperature by warming up the battery by a heater before the battery temperature lowers to a temperature that causes a state in which such problem of the usable input/output power of the battery occurs, is required.
As a battery temperature control device that controls the battery temperature by warming up the battery by the heater, a related art battery temperature control device disclosed, for instance, in a Patent Document 1 has been proposed.
This proposed related art battery temperature control device is a device that performs the temperature control of the vehicle-mounted battery and warms up the battery by the heater when an outside air temperature is lower than a minimum temperature upon OFF-operation of an ignition switch.
Here, the Patent Document 1 discloses, in a drawing, that the battery charge state SOC is inputted to a block that governs the above battery temperature control. And it can be read that when this battery charge state SOC is a setting value or greater, the related art permits the warm-up of the battery.
In the Patent Document 1, however, there is no detailed disclosure nor mention of the setting value of the battery charge state at all. Then, it is plausible that this setting value of the battery charge state is interpreted as a fixed value.
Further, in the case where the setting value of the battery charge state is the fixed value, such a problem explained below arises.
That is, even if a temperature at the ignition switch OFF is a low temperature at which the battery should be warmed up, while the battery charge state is less than the fixed setting value, the warm-up of the battery by the heater is not permitted.
Then, during this state, the battery temperature further lowers, and the usable input/output power further decreases. As a consequence, there arises a problem that a charge current when the vehicle travels up to a battery charging facility and the battery is charged becomes small and thus taking a long time for the charging.